Clostridium difficile is the leading cause of antibiotic-associated diarrhea in the hospital and long term health care settings. In addition to the patient toll, the treatment-associated costs of C. difficile infections to the United States healthcare system have been estimated at $3.2 billion. Although the rate of C. difficile infection in the United States is rising, surprisingly little is known about the mechanisms of C. difficile pathogenesis. C. difficile is believed to be acquired by the host in the form of a dormant spore. To cause disease, the spore must respond in the gastrointestinal tract to signals that trigger germination, thereby allowing growth as a vegetative bacterium, toxin production and subsequent spore formation before excretion into the environment. We have shown that taurocholic acid, a bile acid normally found in the GI tract, and glycine are co-germinants for C. difficile spores. Another bile acid, chenodeoxycholic acid, inhibits taurocholic acid-mediated germination and is toxic for C. difficile vegetative growth. We recently identified the molecular target of bile acids on the C. difficile spore thus identifying the first C. difficile spore germinnt receptor. Our long-term goal is to understand the molecular mechanisms of C. difficile germination and use that knowledge to rationally design inhibitors of C. difficile infection. In ths application, we propose to: (1) identify how the C. difficile spore germinant receptor interacts with bile acids; (2) identify binding partners and the ultrastructural location of the germinant receptor in the C. difficile spore; (3) determine who CspC transmits the bile acid signal to initiae spore germination; and (4) identify other C. difficile germinant receptors. Successful completion of the experiments outlined herein will extend our understanding of the mechanisms of C. difficile germination, open new avenues in the study of C. difficile spore formation and spore germination and lead to the identification of homologs of the newly discovered germinant receptors in other spore-forming bacteria (opening other avenues of investigation).